1. Field of the Invention
The presently disclosed subject matter relates to an autofocus system, and more particularly to an autofocus system equipped with a face recognition and tracking function that causes an AF frame (AF area) that represents the range of a subject brought into focus by autofocus (AF) to track the face of a person that is registered beforehand.
2. Description of the Related Art
A well known television camera system for broadcasting or business use includes an autofocus function that automatically focuses on a desired subject. This type of camera system generally uses a contrast-type AF system. In the contrast-type AF system, the camera system detects the level of contrast in a captured video image, and controls the focus of an imaging optical system such that the contrast becomes the maximum (local maximum).
Normally, a target range of a subject that is brought into focus by AF is not an area of the entire screen of a captured video image, but rather is limited to a particular range that is referred to as an “AF area”. The subject inside the AF area is brought into focus by controlling the focus such that the contrast of the captured image (subject image) inside the AF area becomes the maximum. According to the present specification, a frame that shows an outline of the AF area is referred to as an “AF frame”.
Japanese Patent Application Laid-Open No. 8-334674 and Japanese Patent Application Laid-Open No. 2006-267221 disclose autofocus systems that have an AF frame auto-tracking function that allows the AF frame to automatically track a desired subject that moves inside the image area of a captured video image so that the subject is continuously in focus. The AF frame auto-tracking is performed by detecting the tracking target subject (subject image) in the captured image and moving the AF frame to the detected position. As a method for detecting an image of a tracking target subject from a captured image, Japanese Patent Application Laid-Open No. 8-334674 discloses a method of storing the image of the tracking target subject as a reference pattern and detecting an image that matches the reference pattern from the captured image using a pattern matching processing. Japanese Patent Application Laid-Open No. 2006-267221 discloses a method that detects a subject that moved by determining a difference (difference image) between two captured images that are acquired in succession which are captured images sequentially obtained from a video signal.
Known kinds of AF frame auto-tracking according to types of tracking target subject include: object tracking in which an arbitrary object can be used as the tracking target subject; face detection and tracking in which only a face of a person (or animal of the like) is used as the tracking target subject; and face recognition and tracking in which only a face of a specific person (or animal or the like (omitted hereunder)) is used as the tracking target subject.
The face detection and tracking detects faces of arbitrary people from a captured image by face detection processing and uses one of the detected faces as a tracking target. The face recognition and tracking detects a face of a specific person that has been previously registered from a captured image by face recognition processing, and uses the thus-detected face of the specific person as the tracking target subject.